Prior art ultrasound systems, sometimes referred to as (detection) “pigs”, comprise one ultrasound transducer and a mirror, via which the sound is directed towards the pipe wall. The same transducer receives the backscattered echoes from the pipe wall. This prior art technique has some limitations, e.g.:                the alignment of the transducer has to be nearly perfect; otherwise the echoes will not be detected;        the surface of the pipe wall needs to be smooth; otherwise the signal gets unusable;        cracks and corrosion pitting are not detected.        
EP1707956 discloses a method and system for detecting the depth of cracks in a pipe. The system involves a pig that is configured to send ultrasonic signals circumferentially in the pipeline walls and to receive reflected signals. The document describes an example wherein multiple sensors are located around the pig and arranged to transmit ultrasonic signals that strike the pipeline wall at a certain angle. The surfaces of the pipe wall as well as cracks in the wall act as ultrasonic reflectors that produce reflections at various delay times.
Reflections from the surfaces of the wall repeat at so-called skip distances, dependent on wall thickness. Reflections from cracks can be distinguished because they are received with other delays. The distance to the reflector is determined from the delay time and ultrasonic ray tracing to simulate the path of the reflections. The distance to the reflectors is used to estimate crack depth. The maximum among the depths determined from different sensors is used as estimated depth of the crack.
This known system merely determines the depths of individual cracks. It does not attempt to use an imaging process to form an image of the pipe wall. Furthermore, the known system relies on directional sensors, which produce rays that strike the pipeline wall at a known angle. This makes it possible to use ray tracing to determine the depth.
It has been found that in practice the surface of the pipe wall may be uneven, with erratic thickness variations due to corrosion and wear, not just with isolated cracks. Due to the sizeable sound speed differences between the pipe wall and the fluid within the pipe, the unevenness results in considerable scattering of the ultrasonic rays at unpredictable angles. This would make imaging with the system of EP1707956 very unreliable in pipes that suffer from corrosion and wear.